maybe you should have keep reading before posting this cuz you are making a fool out of yourself..... cuz you can't be more WRONG. the way larian is doing it, is not the way early access game dev does it(in general) I posted a list of some game took at random in my huge early access list and they all did it and still does it with a EXPERIMENTAL branch , you have the stable version of the game and the unstable version of the game the experimental one more uptodate but more bug. it updated regularly (every day) and when it reach a sufficient stable state they push it on the stable version.
i can cite you example if you want but most if not all early access game does it that way. with some exception like Larian who don't have a clue
the way you describe it is very rare among early access dev. in my 73 early access game on steam I could only find 3 games that did not when the experimental branch way. and BG3 is one of them.
keeping back up of previous version can be done manually. or, by not pressing the update button, yes you can do that on steam too. you can disable it all.
and it does not need branching. it's not the proper way to use the BETA patch feature on steam even when you go and read what is the beta patch branching feature from the steam wiki , you can see its not meant to be use as back up version but as a way to provide your player base a way to test upcoming feature before implanting them for good.
and by doing it that way you can retain more active and engaged player who want to give feedback and test the game.
oh and losing your save file , is like a feature on a early access game. don't buy one if you not ready to face bug crash and lost of data.
be we all know by now this BG3 EA is not a EA. the communication is only 1 way.